Epitaxial garnet films

ABSTRACT

A method of making improved garnet thin films in a sputtering apparatus. The film is sputtered from a source containing yttrium or rare earth garnet in bulk form onto a garnet substance. Epitaxial films with square hysteresis loops are obtained only when the garnet substrate is in the (111) orientation.

United States Patent Eric Kay Kenslngton;

Erich Sawatzky, San Jose, both of Calif. 766,698

Oct. 1 l, 1968 Sept. 21, 1 97 1 International Busines Machines Corporation Armonk, N .Y.

inventors Appl. No. Filed Patented Assignee EPITAXIAL GARNET FILMS 3 Claims, 1 Drawing Fig.

US. Cl 204/192,

1 17/62 Int.Cl C23c 15/00 Field 0! Search 204/192,

(Hi) PLANE 5 6] References Cited UNITED STATES PATENTS 3,021,271 2/1962 Wehner 204/192 3,348,962 10/1967 Grossman et al. 1 17/48 3,437,577 4/1969 Kay et a1 204/192 Primary Examiner-John H. Mack Assistant Examiner-Sidney S. Kanter AnorneysHanifin and Jancin and Joseph G. Walsh ABSTRACT: A method of making improved garnet thin films in a sputtering apparatus. The film is sputtered from a source containing yttrium or rare earth garnet in bulk form onto a garnet substance. Epitaxial films with square hysteresis loops are obtained only when the garnet substrate is in the (l 11) orientation.

PATENTEUSEP21 IBII 3 07, 9

INVENTORS.

ERIC KAY ERICH SAWATZKY BY U AJZJ) NEY EPITAXIAL GARNET FILMS CROSS-REFERENCES TO RELATED APPLICATIONS The present application is related to the following copending applications, all of which are assigned to the assignee of the present application. 1. Application Ser. No. 648,811, by Kay and Sawatzky, filed June 26 1967 now U.S. Pat. No. 3,437,577; 2. Application Ser. No. 593,387 by O. Voegeli et al. filed Nov. 10, 1966, now US. Pat. No. 3,514,577.

BACKGROUND OF THE INVENTION The present invention relates to a method of forming thin films, and more particularly to a method for making thin garnet films in a sputtering apparatus.

The preparation of rare earth iron garnet thin films with controlled physical properties is very important. Such films are useful as modulators for light beams, as couplers or modulators in microwave systems, and as the active memory element in beam addressable memory systems. One such beam addressable memory system is shown in the previously referenced copending application, Ser. No. 593,387 (IBM Docket SA9660ll In our prior application Ser. No. 648,811, there was described a method for the preparation of single phase rare earth iron garnet thin films having useful properties. The films of this prior application have controllable properties over large areas, and hysteresis loop characteristics with B,/B, l were observed provided these paramteters were measured over a film area or beam size of about 1 mm? at a time. B, is the remnant magnetic induction and B, is the saturation magnetic induction.

It recently became possible to examine the compensation temperature and coercivities on a much smaller scale, for example, about 5p."', and it became evident that on this microscopic scale a small but intolerable variation in compensation temperature and coercivity exists from bit to bit. This variation interferes with therrnomagnetic writing and readout processes with high bit density, and also makes it difficult to have a sufficiently large signal to noise ratio.

It is an object of the present invention to find ways which give material having very square loop hysteresis curves on a microscopic scale and which have low coercivity requirements at temperatures which are compatible with system requirements such as those described in the previously referred to application Ser. No. 593,387.

This and other objects have been attained by the present invention.

SUMMARY OF THE INVENTION The present invention utilizes conventional sputtering apparatus, for example, a radio frequency field. The substrate is a garnet which is maintained at any temperature less than 500 C. during the deposition operation. The atmosphere in the apparatus during the deposition includes at least percent partial pressure of oxygen. Material is sputtered from a source containing garnet in bulk form and a thin film is formed. After the disposition of the film, it is crystallized at a temperature of between 700 and 1100 C. in a controlled atmosphere. The process here described will be recognized as that of our US. Pat. No. 3,437,577, except that in the present case the substrate temperature may be as high as 500 C.

We have now improved upon that process. We have discovered that it is possible to obtain epitaxial garnet films with square hysteresis loops and temperature stable domain configuration by using the process described above with the added provision that the substrate be a garnet in the (111) orientation. When the substrate garnet has been cut in the (111) orientation there is obtained an epitaxial garnet film with many desireable characteristics for use in beam readable systems. It has been discovered that the use of garnet substrate with the 1 11) orientation is a method of obtaining epitaxy at temperatures as low as 700 C. (Epitaxy has been achieved in the past by certain vapor deposition methods but only a temperatures as high as about 1200 C.). Because these films are obtained at relatively low temperature, they exhibit great resistance to cracking and great strain resistance. Additionally, they have the unexpected advantages of giving square hysteresis loops and showing stable domain behavior over a wide temperature range, both of which are essential features in beam addressable memory applications. Square loop behavior on such a microscopic scale by definition implies magnetic uniformity which proved to be so essential for successful thermomagnetic write and read requirements having a high signal to noise ratio. Attempts on many other single crystal substrates, e.g. Mg0, A1 0 Si0 etc. and on garnet substrates oriented in crystallographic directions other than (11]) were completely unsuccessful.

Stable domain behavior is a very important unexpected advantage of the present invention. In thin single crystal platelets stable domians have in the past been achieved only if they were greater than about 2 to 3 mils in diameter. We have observed temperature stable domains as small as 1 micron in diameter, and it is possible that improved methods of examination would disclose that the diameters are in fact even smaller.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic representation of a garnet, which by definition is a body centered cube, showing the (111) plane along which the substrate should be orientated. The numbers 001, 010, and identify the three principal crystallographic axes in accordance with standard terminolo- 8)" DESCRIPTION OF A PREFERRED EMBODIMENT For applications involving beam addressable memory systems, it is preferred that an iron garnet be deposited on a nonmagnetic garnet substrate.

Iron garnet materials are those described by the formula M Fe 0 wherein M is yttrium or one of the rare earths. Examples of such materials include Gd=Fe -,0, Fe-,0, and Eu Fe 0. These garnets containing iron are ferrimagnetic. Nonmagnetic garnets do not contain iron. Examples are Y Al 0 Y Ga 0 and Gd Al 0 The selection of the particular garnet film and the garnet substrate will depend upon the desired end use. For microwave uses Y -,Fe -,0, deposited on Y Al 0 may be preferred, but for beam addressable memory applications the most preferred combination is a film of od,Fe,o, on a nonmagntic substrate such as Y Al,0, because Gd Fe 0 has a compensation temperature near room temperature.

EXAMPLE The following example is given solely for the purpose of illustration and is not to be construed as a limitation on the present invention, many variations of which are possible without departing from the spirit or scope thereof.

The sputtering apparatus used was identical to that described in application Ser. No. 648,81 1, previously referred to. However, commercially available radio frequency sputtering apparatus such as that produced by Materials Research Corporation,

Model No. 8-3005, could be used. One necessary feature of the apparatus used is the inclusion of a cooling mechanism in the sputtering cathode in order to avoid thermal vaporization of the cathode source. The sputtering apparatus was evacuated by commercially available vacuum equipment capable of producing background pressures of the order of 5 X10 Torr. The sputtering apparatus was activated with a conventional low power (50 watt) radio frequency oscillator and a standard radio frequency power amplifier capable of generating one thousand watts CW output. A frequency of 13,56 megacycles per second was used.

A cathode" sputtering source which consisted of ceramic bulk, gadolinium iron garnet (Gd Fe-fl was used. A single crystal of nonmagneto optically active garnet namely yttrium aluminum garnet (Y Al o was used as a substrate. The substrate is cut from a single crystal of yttrium aluminum garnet such that the plane of the substrate coincides with the (111) crystallographic plane of the single crystal. The surface is prepared by carefully polishing and lapping the as-cut substrate and subsequent ultrasonic cleaning. A (111) crystallographic plane is depicted in the drawing. The substrate was gallium soldered to the temperature controlled counterelectrode in the sputtering apparatus in order to ensure a welldefined substrate temperature during film deposition. The substrate was maintained at 150 C.il C. during the deposition operation. The temperature of the substrate was controlled using the mechanism shown in US. Pat. No. 3,369,989. A set of Helmholtz coils was placed outside of the vacuum chamber in order to provide a magnetic field perpendicular to the dielectric target surface. The superposition of the magnetic field resulted in a significant increase in the deposition rate and it helped to stabilize and confine the glow in the space between the two electrodes within the deposition apparatus. A field of 40 gauss was used throughout the work.

The apparatus was first evacuated to Torr. and he oxygen was introduced in order to create a pressure of X10 Torr. A spacing of 2.5 cm between the source and he substrate was used. The deposition rate was 85A./minute. Films 0.7 microns thick were fabricated.

When first removed from the sputtering chamber, the films were completely amorphous. These amorphous films were converted to single phase epitaxially oriented gadolinium iron garnet films by heating them at 900 C. in an oxygen atmosphere. Tl-le heat treatment was performed in a 2-inches glow bar tube furnace. The tube furnace had a programmable power source, and generally, the heating and cooling rates were 600 C./hr. and 200 C./hr., respectively.

X-ray diffraction, electron diffraction, and X-ray fluorescence were used to determine the presence of crystallographic phases and to check the relative concentarations of Gd and Fe in the films. During the cystallization step, the weight of the films was accurately monitored to within 10 grams using an ultramicrobalance. If the film had (1 a three to five gadolinium to iron ratio by X-ray fluorescence, (2) X-ray diffraction and electron difiraction, clearly indicated gadolinium iron garnet as the only phase, and (3) the films did not gain any weight during crystallization, it was then assumed that the oxygen content did not significantly deviate from the Gd Fe o composition. By the above technique, it was clearly shown that the resulting gadolinium iron garnet films were singlephase, epitaxially oriented films.

Several substrates were used covering an area of approximately 30 sq. cm. Measurements of the resulting films showed that the coercivity varied by less than 1 percent over the entire area and the compensation temperature did not vary by more than 11 C. over the entire area. The chemical and structural properties of the film did not vary over the thickness of the film and microscopic examination showed that the resulting film had no cracks.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the an that the foregoing and other changes in the fonn and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. The method of making single phase, rare earth iron garnet thin films in a sputtering apparatus comprising the steps of:

a. maintaining a garnet substrate having a (1 ll) orientation directed toward the sputtering flux at a temperature less than 500C.

b. maintaining an atmosphere in said apparatus of at least 10 percent pressure oxygen; c. sputtering material from a source containing said rare earth iro n garnet in bulk form to said substrate; d. crystalllzmg the deposited film while simultaneously maintaining said oxygen-containing atmosphere at a controlled reduced pressure to prevent said film from cracking. 2. The method claimed in claim 1 wherein the substrate is nonmagnetic garnet and the film is a ferrimagnetic garnet.

3. The method claimed in claim 1, wherein the substrate is Y Al 0 and the film is Gd Fe ll 

2. The method claimed in claim 1 wherein the substrate is nonmagnetic garnet and the film is a ferrimagnetic garnet.
 3. The method claimed in claim 1, wherein the substrate is Y3Al5012 and the film is Gd3Fe5012. 